Excellent or well-cut diamonds are proportioned so that all of the light entering the diamond is reflected internally, from one mirror-like facet to another, dispersing it evenly through the crown (top) of the diamond. This perfectly balances the white light (brilliance) with intense flashes of fire (dispersion), giving the diamond its sparkle.
While the cut is not perfect, most of the light entering the diamond is still reflected through the top (crown) (the top), producing a level of very high brilliance (white light).
Though it still reflects most of the light, a Good cut displays less scintillation and appears slightly darker with a lack of contrast. Diamonds of this grade are not as brilliant as those of Excellent and Very Good grades.
A Fair-cut diamond, with facets cut only a few degrees out of alignment, can result in light exiting through the bottom of the diamond, known as light leakage, instead of from the top (crown) where it is visible. This creates a diamond with dulled brilliance from poor light performance within the diamond, making the centre of the diamond look dark. For example, if it has been cut too shallow and wide, the diamond will lose light through the bottom. If it has been cut too deep and narrow, the diamond will lose light through its sides.